In the FAQ, population is determined to be how much damage (hit points) the planet can take.

I also see structures that add Defense points to the planet (surface bunker = Defense +20).

Are these related? Is 200 population better or worse than a surface bunker. Or they completely different things?

Population = HP of the planet. The more population a planet has, the more energy the enemy has to burn to get it down to <10 population for invasion. The max cap regardless of how high your attack is, is 20 population per hit.

Defenses = Damage reduction, meaning fewer population lost when a ship attacks. However, defenses are worthless aganist ships with 200 - 300 more attack than your defense will do the 20 max damage per hit no matter what.

In short, if you had to choose between the two, pick population. It's better as defense than defense buildings because of the "20 population killed per hit" cap regardless of how high the attacking player's attack power is.

Yeah, population means alot more than defense but don't forget.. Defense also has other impt factor: invasion resistance..